1. Field of the Invention
The present invention refers to a method for compensating the non-linear distortions of high-frequency signals, especially when observing the amplitude modulated signals with narrow band receiver. The invention refers also to a device for carrying out said method.
2. Description of the Related Art
A device known as digital radio has been long used in processing high-frequency signals. The latter divides the processed signal to two parts and mixes them with internally generated sine and cosine reference signal of the same frequency as the processed signals. The resulting signals are known as I and Q branches. Thus the information hidden in the carrier frequency is brought to the baseband by the square amplitude demodulation.
Sampling frequency at such way of processing signals is usually chosen to be an integer multiple of the frequency of the input signal. But this may cause problems because of the non-linearity of the analogue-digital converters and other electronic components. This non-linearity can be described as an additional amplitude modulation of the input signal, resulting in the higher harmonic signals appearing in the frequency band of interest and distorting the amplitude measurements of the observed signal. Additionally, the integer ratio between the sampling frequency and the input frequency means that the analogue-digital converter is operating in only N points and not in its entire operational range. Thus, the period of the additional modulation repeats after every N points.
The said problem was partially solved by L. Doolittle (L. Doolittle, H. Ma, Digital Low-Level RF Control Using Non-IQ Sampling, LINAC 2006) who proposed such sampling frequency change that the ratio between the latter and the frequency of the input signal is a rational number. Once chosen, the said ratio remains unchanged. In this case the higher harmonic frequency signals move away from the frequency of interest and the analogue-digital converter operates in more points but still not in its entire range. If the analogue-digital converter operates in more points this means that the frequency of repeating of the additional modulation diminishes. There are more higher harmonic components in the output signal but the power of a single component is lower (the overall power of the error always remains the same).